Over the last fifteen to twenty years or so, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little or no further maintenance throughout the expected life of the battery. This type of battery, often termed a "maintenance free battery", was first commercially introduced in 1972 and is currently in widespread use.
There has been a considerable amount of attention addressed to the type of alloys used in maintenance-free batteries. When the maintenance-free batteries were first commercially introduced, conventional automotive lead-acid batteries commonly used grids formed from antimony-lead alloys in which the antimony content ranged from about 3-4.5% by weight of the alloy composition. Such alloy compositions were capable of being formed into battery grids by gravity casting techniques widely used in the 1970's. Moreover, the batteries made using grids of those alloy compositions had desirable deep discharge cycling characteristics.
Unfortunately, such high antimony content lead alloys could not be used for grids for maintenance-free batteries. The use of such alloys resulted in batteries having undesirable gassing characteristics. In other words, grids made from such alloys accepted an excessive current during constant voltage overcharge so that excessive gas generation occurred. Accompanying this gas generation was the loss of water from the sulfuric acid electrolyte. Much commercial interest for alloys for maintenance-free batteries centered around calcium-tin-lead alloys and "low antimony" lead alloys, i.e., the antimony contents in such alloys being in a range of about 1-2% by weight or so.
In conventional lead-acid battery construction, a strap is cast onto the lugs located on the battery plates to electrically connect the plates of the same polarity together. This cast strap typically includes a portion, often termed a "tombstone" because of its shape, which is positioned adjacent to an aperture in the battery container cell partition. Adjacent tombstones and associated straps which connect plates of opposite polarity are initially assembled on either side of the aperture and are then welded to form an intercell weld in a through-the-cell partition configuration. This intercell weld then serves as the current path from one cell of the battery to the adjacent cell. As used herein, the term "strap" or "battery strap" refers to the strap connecting the lugs of the respective battery plates as well as the portion used to form the intercell connection.
Whether the lead-acid batteries were of a maintenance-free type or not, the intercell connection has been of substantial concern to battery manufacturers. Thus, a considerable amount of technology has developed over the years in an effort to provide a reliable, through-the-partition intercell connection.
One type of technology has been termed an "extrusion-fusion" welding process. In this process, both the adjacent cell tombstones are first extruded under cold metal flow conditions into the aperture in the cell partition. The extruded tombstone portions are then fused using electrical resistance heating to form a weld nugget completely filling the cell partition or aperture. Many other techniques are known for forming the intercell connections, among these being processes in which the intercell weld is created principally or solely by fusion.
Crucial to any of the processes by which the intercell connection is made is the need to have an electrolyte-tight seal between the portion of the strap forming the tombstone weld nugget and the partition wall. Such a tight seal is needed for many reasons. It is thus desired to prevent any path for electrolyte from one cell to another that would create, in effect, a minor short-circuit path. Without cell-to-cell electrolyte isolation which would be compromised by even a minor short circuit path, the desired and correct maintenance of the battery voltage is likewise compromised. Additionally, and importantly, when intercell welds corrode and fail, the potentiality for explosions exists as is well known.
Maintaining the electrolyte-tight seal throughout a satisfactory battery service life is quite difficult. Thus, the intercell weld nugget is typically submerged to some extent in the electrolyte. Accordingly, intercell corrosion problems can become a significant concern.
It is, of course, well recognized that lead-acid batteries are perishable products. Eventually, such batteries will fail; and there are several possible failure modes, e.g., due to positive grid corrosion. The thrust of maintenance-free batteries has been to forestall the failure in performance for a period of time commensurate with the expected life of the battery, e.g., three to five years or so. However, for the reasons evident from the foregoing, it is highly desirable, if not perhaps essential, to have the eventual failure mode to be other than failure due to faulty intercell connections.
In the past several years, there have been several factors which have complicated the situation. One is seemingly ever-increasing power and energy requirements for SLI automotive batteries. Many factors have contributed to the need and/or desire for batteries having more power.
Yet another complicating factor is the "under-the-hood" space requirements. Automobile manufacturers have lessened the space available for the batteries. Typically, it has become necessary to provide lower profile batteries, i.e., batteries having a less overall height than previously used.
These complicating factors of increasing power and less available space have required battery manufacturers to alter the internal configuration and designs to provide the power and energy needed in the desired low profile container. This has typically involved increasing the number of plates per cell and decreasing the thickness of the battery grids. For example, the number of plates in a BCI Group 24 battery over the past several years has increased from about 13 to about 19 or so, while the thickness of the positive grids has decreased from about 70-75 mils down to 55 mils, and even 45 mils or so. This has allowed battery manufacturers to provide batteries having relatively high rated capacities.
What has also occurred in recent years for various reasons is a substantial increase in the vehicle under-the-hood temperature to which an automotive SLI battery is exposed. This increased temperature obviously presents a particularly acute situation in the warmer climates. One battery manufacturer has perceived that, in the recent past, the temperature in such warmer climates to which an SLI battery in service is exposed has risen from about 125.degree. F. to about 185.degree. F. in new automobiles, or even more.
The specific temperature increase to which SLI batteries are now exposed is not per se of particular importance. What is important is that the under-the-hood temperatures have in fact increased. The impact of this rise in vehicle under-the-hood temperatures on the failure modes and the timing of such failures has been substantial. The incidence of premature battery failure due to failure of intercell welds has been significant. The industry has failed to appreciate the impact of all of these complicating factors on current maintenance-free battery designs and their performance and useful service life.
One attempt to deal with the acute problem of the high under-the-hood temperatures has been to retrench. Thus, one automotive battery manufacturer has developed a battery specifically directed for use in high heat environments in which thicker positive grids are used, less plates per cell are used and the head space in each cell is filled with hollow plastic microspheres. The presence of such microspheres may perhaps be perceived to function as a vapor barrier to electrolyte to minimize evaporative loss of water in the electrolyte or for limiting heat transfer or for perhaps some other purpose.
A wide variety of strap alloys have been used over the years in maintenance-free and in other SLI battery applications. More typically, these lead-based alloys include antimony, arsenic and tin in a wide variety of levels together with other alloying ingredients such as copper, sulfur and selenium. Typically, the antimony content has ranged from about 2.7 to about 3.4% by weight of the total alloy. One prior alloy of this general antimony content also included, arsenic in the range of 0.13-0.2%, tin in the range of 0.3-0.4% and selenium in the range of 0.013-0.02%. Another antimony-lead alloy of this type also included arsenic in the range of 0.16-0.19%, tin in the range of 0.14-0.16% with copper in the range of 0.05-0.06% and sulfur in the range of 0.0007-0.0017%. Still another antimony-lead alloy used in an SLI automotive battery included arsenic at a level of 0.07%, tin at 0.06% and copper at 0.037%. Lastly, still another strap alloy of this type used in an SLI automotive battery included arsenic at a level of 0.005%, tin at a content of 0.005%, selenium at 0.008%, copper at 0.003% and sulfur at 0.0006%.
U.S. Pat. No. 5,169,734 to Rao et al. disclosed a lead-based alloy that imparted to an intercell weld the desired mechanical characteristics that resulted in substantially enhanced corrosion resistance in actual service life. Indeed, batteries made using strap alloys in accordance with the '734 Rao et al. patent provided substantial improvements in service life in comparison to batteries made using prior strap alloys.
The '734 patent is predicated on the discovery that a major failure mode of intercell welds is due to the buildup of a corrosion layer, believed to be at least principally of lead sulfate on the negative tombstone and lead dioxide on the positive tombstone. The corrosion process is a natural occurrence in a lead-acid battery; and the rate of this corrosion is greatly influenced by battery service temperature, grid chemistry and method of strap production, and strap alloy chemistry, among other factors. It is the intent of the battery designer to restrict the corrosion rate to an acceptable, controlled rate so that the intercell connections of the battery do not fail prematurely in service.
In conjunction with the '734 invention, it was discovered that a corrosion layer builds up on the tombstone face between the cell partition wall and the adjacent tombstone face of the intercell weld. This buildup occurs on the tombstone face on both positive and negative polarity tombstones of the adjoining cells. As the buildup occurs, the corrosion layer is believed to function as a wedge, forcing the face of the weld out of electrolyte-tight contact with the cell partition. This wedging action causes stress fractures or cracks and propagates such fractures and cracks through the intercell weld nugget, or around the weld through the tombstone parent metal matrix ultimately leading to failure of the intercell connection.
In view of this wedging action, Rao et al. discovered that the strap alloy employed must impart to the resulting intercell weld a unique set of mechanical properties for the intercell weld to survive the high temperature conditions which are present in current automobiles when operated in the warmer climates. More specifically, it was found that the strap alloy used to make the intercell connection must impart to the resulting intercell weld adequately high toughness or higher total energy required to fracture the material while having satisfactory ductility.
The lead-based alloys in the '734 patent that impart to an intercell weld these desired mechanical characteristics have the following composition, all of the percentages being based upon the total weight of the lead alloy: antimony in the range of from about 3.0-3.3% or so, arsenic in the range of from about 0.04 to 0.07% or so, tin in the range of from about 0.04 to 0.07%, and selenium in the range of from about 0.014 to 0.02%. The changes in the composition of alloys of this type from those previously used, Rao et al. note, may appear superficially to be somewhat subtle, but these changes impart substantially different results in terms of corrosion resistance in actual service life.
Further, Rao et al. determined that the intercell connection failure mode observed in batteries subjected to continuous 155.degree. F. exposure (sometimes termed "hot box" exposure) were found to be very similar to the intercell connection failures observed in batteries removed from vehicle service due to premature failure. Accordingly, such hot box exposure was considered to provide an accelerated and reliable laboratory test to prove the adequacy, and the method for evaluating the integrity of, the intercell connections in lead-acid batteries. According to the Rao et al. '734 patent, satisfactory intercell connections should be capable of reliably withstanding the 155.degree. F. hot box exposure for at least 15 weeks without the appearance of cracks visible in photomicrographs at 10.times. amplification. More preferably, Rao et al. state that intercell connections should be capable of reliably withstanding at least 20 weeks of hot box exposure at 155.degree. F., viz., essentially all of the intercell connections will not show cracks in essentially all of the batteries tested. In other words, out of 100 batteries tested, Rao et al. indicate that "reliably withstanding" such exposure should result in no more than one battery or so that fails through a faulty intercell weld due to intercell corrosion.
One dramatic breakthrough which has been made is reflected in U.S. Pat. No. 5,298,350 to Rao. The Rao '350 patent thus discloses low calcium-tin-silver lead alloys for forming positive grids which impart to the resulting battery substantially enhanced resistance to positive grid corrosion, particularly when the battery is exposed to the relatively high under-the-hood temperatures of current automobiles in warmer climates. Indeed, the decrease in premature battery failure due to positive grid corrosion achieved when the Rao alloys are utilized to make the positive grids is remarkable.
This breakthrough, in effect, has extended the time window for the service life of automotive batteries, lessening the grid growth that can contribute to weld failure and making the service life achievable from the intercell weld ever so more important. The strap alloys used in forming cast-on straps and the intercell welds in the Rao et al. '734 patent certainly provide desirable weld life; yet, as the service life requirements become ever more demanding, yet still further improvement is needed.
It is accordingly an object of the present invention to provide a maintenance-free, lead-acid battery capable of satisfactory service life even when exposed to the relatively high temperature under-the-hood environment in the automobiles which have been manufactured in the last several years.
Another and more specific object lies in the provision of an alloy composition that may be used for making the straps for such maintenance-free batteries.
A still further object provides a strap alloy for such batteries that imparts to the batteries enhanced resistance to corrosion in comparison to alloys presently being used.
Yet, another object of the present invention is to provide an SLI automotive lead-acid battery in which the eventual principal battery failure mode is a mode other than faulty intercell welds.
Other objects and advantages of the present invention will be apparent as the following description proceeds, taken in conjunction with the accompanying drawings.